Theoretical investigation of triscarbazole derivatives as host materials for blue electrophosphorescence: Effects of topology

We have investigated the electronic structure of triscarbazole derivatives used as host materials in blue phosphorescent organic light-emitting diodes. The results of density functional theory calculations show that, in the case of triscarbazole derivatives where the carbazole units are linked via C-C bonds, the frontier molecular orbital energies are modulated by strong molecular orbital interactions between the central and side carbazole units. On the other hand, in the case of triscarbazoles linked via C-N bonds, the combination of inductive effects and molecular orbital interactions tunes the frontier level energies and, interestingly, gives rise to an ambipolar character. In the C-N linked systems, the lowest triplet states are characterized mainly by an electronic transition localized within the central carbazole, while in the C-C linked compounds it is the longest oligo-para-phenyl segment to be found in the chemical structure that defines the lowest triplet transition. When the N-H group of the central carbazole unit is replaced by other groups [O, S, CH ₂, C(CH₃)₂, C(CH₃)(CF₃), and C(CF₃)₂], the HOMO/LUMO energies fluctuate substantially in the absence of the side carbazoles, but these variations are significantly reduced in their presence; also, the singlet-triplet energy differences decrease substantially when going from the isolated central unit to the triscarbazole-like derivatives.